Early-strength and/or high-strength concretes are also referred to as high-performance concretes that either set or cure quickly (quick-setting concretes) or exhibit relatively high strengths and are particularly resistant toward aggressive media (high-strength concrete).
Quick-setting concretes are generally produced using quick-setting cements derived from particular clinkers comprised substantially of calcium sulfoaluminates, high-alumina cements, or mixtures of high-alumina cement with portland cement. These particular clinkers, may have its early strength be generally controlled by means of organic additives. Such organic additives, which are foreign constituents in these mineral mixes, can have undesirable side effects and react uncontrollably in the case of temperature differences and/or raw material fluctuations; exhibit adverse effects, such as slow solidification and curing; or even cause failure in regards to solidification and curing. Thus quick-setting cement is therefore relatively unsuitable for the production of ready-mixed concrete.
Another type of quick-setting cement uses a hydraulic binder, which is capable of flow and solidification after the addition of water, together with an accelerator component that functions to accelerate setting. The accelerator component is normally very fine calcium hydroxide particles having particular specific surface areas and/or particle sizes, as shown in International Publication No. WO 2006/111,225A1, the entire contents of which are incorporated herein by reference. In this document, the prior art in the field known at the time is summarized and it is indicated that the accelerated formation of calcium silicate hydrate phases associated with the calcium silicate in the cement particles is critical for the establishment of strength. Accordingly, the fine calcium hydroxide particles that are added should accelerate the calcium silicate hydrate formation (CSH formation) commencing after about 6 to 8 hours by nucleation.
In the case of quick-setting cements containing calcium hydroxide as an accelerator component, such as described in WO 2006/111,225A1, there is a risk of unsatisfactory storage stability. It is known that calcium hydroxide undergoes carbonate formation in the presence of atmospheric carbon dioxide. The reaction behavior of these quick-setting cements is consequently not constant. As a result, the high early strength potential cannot always be exploited.
To obtain high-strength and particularly resistant concretes, it is usual to use low-C3A portland cements of the strength class 42.5 or 52.5 R in combination with microsilica. The microsilica is collected as fly dust in ferrosilicon production and comprises, for example, from 85 to 98% by weight of amorphous SiO2. The objective is to achieve very dense packing of spheres in the dry phase which then, after addition of water, also leads to a cement paste in which the particles are closely packed. In addition, the microsilica is supposed to react with the calcium hydroxide (Ca(OH)2) that is liberated after about 6-8 hours from the reaction of the cement clinker phases, (e.g., calcium silicate phases, in particular C3S) or secondary constituents with water.
While the hydrate phases are formed from, for example, the C3A and C3S of the clinker particles, microsilica and Ca(OH)2 form additional C—S—H phases which then expand into the voids that are still present, thereby, making the cure hardened cement matrix denser. As a result, this pozzolanic reaction between the microsilica and the Ca(OH)2 forms particularly dense and thus resistant and durable concretes that may sometimes exhibit an extremely high compressive strength. Disadvantages of the use of microsilica (e.g., silica dust from iron silicide production) include the concrete color, which is usually too dark and nonuniform for visible concrete, and the necessity of installing and operating a separate costly and complicated metering facility.
The reaction principle of the pozzolanic reaction between, for example, microsilica and Ca(OH)2 is shown schematically in FIGS. 1a, 1b, and 1c. Initially, a dry mixture of cement particles 1 and microsilica particles 2 is present (FIG. 1a). After mixing with water and solidification, there is the delay phase known to those skilled in the art during which the mixture does not undergo any appreciable or further curing. Only after, for example, about 6-8 hours does a curing reaction of the silicate and aluminate phases with water occurs that forms, for example, acicular CSH phase crystals 3, resulting in the liberation of Ca(OH)2 4 (FIG. 1b). The Ca(OH)2 reacts with the microsilica particles 2 and additionally forms fine C—S—H phase crystals 5. These crystals densify the structure of the cure hardened cement and by means of this pozzolanic reaction produce a denser microstructure exhibiting higher strength and durability (FIG. 1c).
Not only microsilica, but also other SiO2 components, such as silica dust, nanosilica, metakaolin, or fly ash can be used for the known pozzolanic reaction.
Another technology for producing high-performance concretes, which is likewise based only on mineral material and makes an increase in early strength possible without requiring pozzolanic reactions, is based on binders that are optimized only in terms of particle size. In this technology, use is made of normal cements in combination with superfine cements. The production of superfine cements is known, for example, from European Patent No. EP 0 696 558 B1, the entire contents of which are incorporated herein by reference. These superfine cements have a particle size below 20 μm and can be produced economically on an industrial scale down to particle sizes of 2 μm.
Upon utilizing this technology, formulation-dependent high-performance cements such as those in high-early-strength concretes; cements for high-strength concretes, preferably high-strength visible concretes; and cements having a particular resistance, e.g. toward aggressive media, can be made. These high-performance cements are standard cement formulations that exhibit extraordinary properties with additives only required to adjust for processability.
FIG. 2 schematically shows the principles associated with this technology. The relatively coarse particle 6 of the normal cement may have a diameter (d95) in the range from 25 to 30 μm. A d95 particle diameter means that about 95% of the particle diameters measured by volume percent has a value within the specified range. In the interstices between the coarse particle 6, the relatively small particles 7 of the superfine cement may have a particle size diameter (d95) in the range from 2 to 20 μm. A supplementation of and an increase in the early strength exhibited by this cement after 6-8 hours can according to this technology be achieved by the addition of Ca(OH)2 (see WO 2006/111,225A1 or finely divided silica.
This technology is limited in that the final strength of the cement is not increased (see WO 2006/111,225A1, table 1: compressive strengths after 28 days), rather the early strength of the cement is only established earlier and capable of being regulated.
It is an object of the invention to provide hydraulic storage-stable binders based on only mineral materials, which can achieve high strengths earlier and exhibit increased end or final strengths as compared to known hydraulic binders.